(also polynucleotides), major biopolymers that are found in every cell of every living organism. Nucleic acids were discovered in 1868 by the Swiss scientist F. Miescher in the nuclei of cells that were isolated from pus and from salmon sperm; later, these biopolymers were also found in the cytoplasm. Two main kinds of nucleic acids are distinguished: deoxyribonucleic acid (DNA), which is found mostly in the nucleus, and ribonucleic acid (RNA), which is found mostly in the cytoplasm.

Nucleic-acid molecules consist of long polymer chains with a molecular weight ranging from 2.5 × 10⁴ to 4 × 10⁹. Each chain is constructed from monomeric units called nucleotides in such a way that the hydroxyl groups at the 3′ and 5′ carbon atoms in the carbohydrate portions of adjacent nucleotides are bound by a phosphate group. Ribose is the carbohydrate component of RNA, while the nitrogen components are the purine bases adenine and guanine and the pyrimidine bases uracil and cytosine. Deoxyribose is the carbohydrate component of DNA, and uracil is replaced by thymine, or 5-methyluracil. The phosphate radical and the carbohydrate are the nonspecific portion of a nucleotide, and the purine or pyrimidine base are the specific portion. Most nucleic acids also contain small quantities of minor bases—usually methylated purine and pyrimidine derivatives. Nucleic acid chains contain from tens to thousands of nucleotides, arranged linearly in a definite sequence that is unique to each nucleic acid. Thus, both RNA and DNA consist of a huge number of individual compounds. The linear sequence of the nucleotides determines the primary structure of a nucleic acid. The secondary structure of a nucleic acid results from the mutual, complementary attraction of certain pairs of bases. Specifically, guanine is bound to cytosine and adenine to uracil or thymine by hydrogen bonds and by hydrophobic interactions.

The biological function of the nucleic acids is to store, determine, and transmit hereditary information. This information is recorded in the form of a sequence of nucleotides—the genetic code. The replication of DNA occurs during mitotic cell division; as a result of replication, each daughter cell receives the same amount of DNA and thus contains a program for developing all the characteristics of the parent cell. In order to convert this genetic information into specific characteristics, RNA molecules are synthesized on a template of DNA molecules—a process called transcription—and subsequently proteins are synthesized with the participation of several kinds of RNA—a process called translation.

Great advances in molecular genetics and molecular biology were made possible as a result of research conducted between the early 1950’s and the 1970’s on the structure and function of nucleic acids. The most important achievement in the study of the chemistry and biology of the nucleic acids was the double-helix model of the DNA molecule, which was proposed in 1953 by J. Watson and F. Crick. This model helped explain many of the properties and biological functions of DNA.

Nucleic acids have also been found in organelles, for example, chloroplasts and mitochondria, where their functions are now under study. Comparative analysis of nucleic acids in different groups of organisms plays an important role in solving some problems in taxonomy and evolution. Every species of organism carries its own characteristic type of DNA and RNA. The degree of structural resemblance between nucleic acids of different species is indicative of the phylogenetic similarity of the organisms.